It is known to use so-called double converters for transmission of electric power between an alternating-voltage network and the direct-voltage side of the converter. Such a converter usually comprises two line-commutated controllable converters, for example three-phase thyristor bridges, which are antiparallel-connected on their dc side and which have their ac terminals connected to an alternating-voltage network. In a double converter, only one of the two converters at a time is operating. A double converter can transmit power in an arbitrary direction between its ac side and its dc side and do this at an arbitrary polarity of the direct voltage or of the direct current.
In double converters, large circulating short-circuit currents may arise if, by mistake, valves in both the converters become conducting simultaneously. These currents may cause serious damage to the equipment before any overvoltage protection device and circuit breaker on the ac side have time to react. It has therefore been necessary hitherto to provide double converters with fuses or with impedance elements such as inductors or resistors. These measures have obvious disadvantages in the form of, for example, increased complexity, increased manufacturing cost, reduced reliability in operation, and increase of voltage drops and power losses.
The direct-voltage network connected to the direct-voltage side of a double converter may be of different kinds. It may consist of one single load object, for example a dc motor, or of a group of dc motors. It may consist of a direct-voltage network, to which a plurality of load objects are connected or may be connected. An example of such an application is a railway catenary supply system intended to supply direct-voltage vehicles. According to a further alternative, the double converter may form part of a frequency converter with a direct-voltage intermediate link, for example a catenary supply system intended to supply ac vehicles.
In several of these applications, the direct voltage on the direct-voltage side of a double converter has a constant polarity (and often also approximately constant magnitude). One of the two converters of a double converter will then only operate as a rectifier and the other converter only as an inverter, and which of the converters is active at a certain time is determined by the currently prevailing direction of the power flow between the alternating-voltage network and the direct-voltage network.